1. Field of the Invention
The present invention relates to a coating composition containing ceramic components, for preventing high temperature oxidation and which is useful when applied to graphite electrodes employed in electric furnace steelmaking.
2. Description of the Prior Art
Heretofore, it has been attempted to prevent high temperature oxidation of graphite electrodes used in electric furnace steelmaking, by coating it with a special paint.
For instance, a paint for preventing oxidation of graphite electrodes has been known from Japanese Patent Publication No. 25256/1979. The paint consists of a base powder, silica, a fluoride (or a powdery low melting component) and a dispersion aid. However, this oxidation preventing paint has practically little effect due to the occurrence of severe scaling off of the coated layer. Indeed this paint, as shown with a comparison test, results in about 80% of the coated layer placed on a graphite electrode falling off after the first charge (after about two hours' operation of the electrode; see Comparison Example 3 given below). Higher permeability and adhesion together with higher heat resistance and higher throwing power are required for these paints, in order to persist against thermal shocks, since the graphite electrode will very often encounter sudden temperature changes with temperature differences varying in a wide range during operation.
Previously a heat radiative ceramic coating composition exhibiting a heat resistivity of over 1,850.degree. C. and excellent adhesion, for use in refractory internal walls of industrial heating furnaces and for metal constructions in furnaces, was proposed by our prior Japanese Patent Application No. 187,695/1981. This ceramic composition consists of the following three components:
(a) 40-75% by weight of silicon carbide as heat radiation component, PA0 (b) 15-40% by weight of a heat radiation promoting and binding component consisting of PA0 (c) 10-35% by weight of an additive for increasing the adhesion and binding strength between the coated layers, consisting of PA0 (a) 40-75% by weight of silicon carbide as a heat radiation component; PA0 (b) 15-40% by weight of a binding heat radiation promoting component consisting of PA0 (c) 10-35% by weight of an additive for improving the adhesion to the graphite electrode and increasing the binding strength between the coated layers, consisting of PA0 (d) 5-20% by weight of a metal powder consisting of PA0 (e) 2-5% by weight of a sintering promoter mixture consisting of PA0 (f) 3-7% by weight of a melting point lowering component consisting of
3-20 parts by weight of silicon nitride, PA1 5-20 parts by weight of salt of phosphorous-containing acid, PA1 2-10 parts by weight of chromium oxide, PA1 2-10 parts by weight of tantalum carbide, and PA1 5-20 parts by weight of pulverous aluminum, and PA1 1-10 parts by weight of aluminum oxide, PA1 3-15 parts by weight of glass powder, PA1 3-15 parts by weight of zirconium oxide, PA1 1-10 parts by weight of silicon dioxide, PA1 1-10 parts by weight of magnesium oxide, and PA1 1-10 parts by weight of iron oxide. PA1 3-20 parts by weight of silicon nitride, PA1 5-20 parts by weight of a salt of phosphorouscontaining acid, PA1 2-10 parts by weight of chromium oxide, PA1 2-10 parts by weight of tantalum carbide, and PA1 5-20 parts by weight of pulverous aluminum; PA1 1-10 parts by weight of aluminum oxide, PA1 3-15 parts by weight of glass powder, PA1 3-15 parts by weight of zirconium oxide, PA1 1-10 parts by weight of silicon dioxide, PA1 1-10 parts by weight of magnesium oxide, and PA1 1-10 parts by weight of iron oxide; PA1 0-40 parts by weight of pulverous copper, PA1 0-40 parts by weight of pulverous nickel, PA1 0-40 parts by weight of pulverous stainless steel, PA1 0-40 parts by weight of pulverous iron, and PA1 0-40 parts by weight pulverous tin; PA1 10-30 parts by weight of silver carbonate, and PA1 30-50 parts by weight of copper sulfate, and/or PA1 30-50 parts by weight of iron sulfate; and PA1 30-60 parts by weight of iron fluoride, and PA1 40-70 parts by weight of copper fluoride,
Using this heat radiative ceramic coating composition however, one was not able to attain a coating layer having very high gas-tightness (required for the graphite electrodes). This coating composition, as will be shown afterwards in the Comparison Examples will scale off to an extent of 60-80% after two or three charges in operation of the electrode.
The present invention provides an excellent coating composition for preventing the high temperature oxidation of graphite electrodes, and which will provide a steelmaking graphite electrode with a burnt coated layer exhibiting excellent adhesion and superior gas-tightness.